The testing of semiconductor wafers and other types of integrated circuits (ICs), collectively known as devices under test (DUTs), needs to keep pace with technological advances. Each IC has to be individually tested, typically before dicing, in order to ensure that it is functioning properly. The demand for testing products is driven by considerations of new chip designs and higher volumes.
In particular, chips are getting smaller and they have more tightly spaced contact pads. The pads are no longer located about the circuit perimeter, but in some designs may be found within the area occupied by the circuit itself. As a result, the spacing or pitch of probe arrays required to establish electrical contact with the pads or bumps is decreasing. In addition, the requirements on planarity are increasing.
To address the increasingly fine pitch of the pads, several prior art solutions have resorted to using more effective space transformers for achieving pitch reduction and improved probe performance. Several examples of recent space transformers for probe contact assemblies are found in the patent literature, including U.S. Pat. Nos. 6,676,438 and 6,917,102 to Zhou et al. These teachings include several contact structures and production methods, including an embodiment using the flip-chip bonding process to attach the space transformer. Still other prior art, such as U.S. application Ser. No. 2006/0033516 to Rincon et al. teaches a probe and universal tester contact assemblage that is capable of testing multiple chips. This assemblage employs a wire bond between the end of a trace that is connected to a probe and a fan-out trace that belongs to an intermediate element or interposer made of a dielectric material to step up from low pitch contacts to fine pitch contacts.
In addition to the finer pitch of the pads to be tested, there has also been a move to increase the test signal frequencies. Some prior art probe devices address the coupling and noise problems arising at such higher frequencies. For example, U.S. Pat. No. 6,714,034 discusses the inclusion of an inductive (magnetic) filter to suppress AC noise on the DUT ground and U.S. application Ser. No. 2005/0012513 to Cheng et al. teaches a probe card assembly with a stiffener ring and a set of coaxial transmitters. Furthermore, U.S. application Ser. No. 2005/0099191 talks about the design of a multi-GHz probe structure for optimizing the signal path to improve bandwidth. Unfortunately, this probe structure can only address a limited number of pads and is thus only suitable for devices with a small number of pads located along the die perimeter. Devices that have a large number of fine pitch pads located along the perimeter or arranged in an array cannot be tested with such probe structure.
Clearly, as more complicated arrays of pads having a fine pitch are to be tested at high frequencies the prior art space transformers and high-frequency handling can not be integrated to produce suitable solutions. In particular, the high-frequency performance of typical wired space transformers that employ via holes and copper wires that use heads equipped with buckling beam or similar probes is insufficient. Even though probe cards employing such space transformers can be made cheaply and relatively quickly, they experience unacceptably high levels of electrical cross-talk and self-inductance with an upper bound on test frequency of about 0.9 GHz at a 3 dB bandwidth. Furthermore, such probe cards can only be made with up to about 1,500 connections, since the wiring process becomes too cumbersome at higher connection counts.
Some probe cards avoid the use of wires and employ instead a space transformer made of an organic substrate (e.g., MLC or MLD) with solder reflow connections to a connection board (typically a printed circuit board to which the test signals are applied). Such probe cards exhibit much improved high frequency performance, typically up to 3 GHz at a 3 dB bandwidth, but are hard to make and require expensive lithographic processing.
Employing wire bonding techniques and wire bonds is in general a very low-cost and mature method of making electrical connections and is used in various related contexts including interposers, probes and space transformers. For example, U.S. application Ser. No. 2002/0125584 discusses using wire bonding to ensure high bond strength between the bonding pads and conducting wires when wire-bonding chips to carriers. Specifically, the weak bonding strength problem is solved by designating a separate section of the IC pad for probing and a separate section for wire-bonding. U.S. application Ser. No. 2002/0194730 teaches how to repair probes mounted on a space transformer that were shaped and made using wire-bonding. U.S. application Ser. No. 2003/0116346 shows how to use a wire-bonding machine to make stud-bumped probes.
Although the prior art solutions individually address some of the problems associated with pitch step up and reliable connections, there is no apparatus or method that combines the requisite characteristics in a single space transformer that can be used in a probe card or testing apparatus. Specifically, what is needed is a space transformer that is compatible with high frequency test signals, easy to make, low cost and can address densely packed pads or bumps arranged in arrays.